1. Field of the Invention
The field of the present disclosure is that of the fiber-optic ring interferometers. More precisely, the invention relates to an interferometric measurement system based on a plurality of fiber-optic ring interferometers. A particular application of such a system relates to a device using several fiber-optic gyroscopes (FOGs) for measuring rotations about several axes.
2. Description of the Related Art
As known, a FOG is based on a Sagnac ring interferometer in reciprocal conFIG.uration, the ring being formed by a fiber-optic coil. A fiber-optic gyroscope generally includes a broad-spectrum source, a first beam splitter (called source-detector separator), a single-mode spatial and polarisation filter at the common input-output of the interferometer, a second splitter (called coil splitter), a fiber-optic coil and a detector.
In the present document, it is meant by broad-spectrum source, a light source having a coherence length lower than one millimeter.
In a reciprocal conFIG.uration, after having passed through the polariser and the spatial filter, the incident light beam is split to produce two secondary beams. The two secondary beams, polarised linearly at the input, propagate in opposite directions along a closed optical path, recombine with each other and produce interferences that depend on the phase shift of the beams during their recombination. At rest, the optical paths are perfectly reciprocal relative to each other, and the phase shifts induced in the propagating direction and in the counter-propagating direction cancel each other.
It is known that a Sagnac ring interferometer is sensitive to the physical phenomena liable to produce non-reciprocal phase-shifts. Among the main physical phenomena, the rotation of a Sagnac ring interferometer, with respect to an axis perpendicular to the surface of the ring, induces a phase-shift proportional to the rotational speed Ω. From this property, called the Sagnac effect, follows the main application of a Sagnac ring interferometer to a gyroscope for measuring a rotational speed. Due to their stability, reliability and compactness, the fiber-optic gyroscopes are more and more used for rotation measurements in the inertial guidance or navigation systems.
Other physical phenomena, such as the Faraday magneto-optic effect, are also liable to induce non-reciprocal phase-shifts.
However, some phenomena produce effects liable to disturb the accuracy, the sensitivity and the time stability of a fiber-optic gyroscope. Hence, polarisation effects in fibers may induce spurious phase-shifts at the origin of bias errors. The use of a broad-spectrum source, a high extinction-ratio common input-output polariser, and a polarisation-maintaining optical fiber in a fiber-optic coil is advocated so as to limit the bias errors of a FOG (H. C. Lefèvre, J. P. Bettini, S. Vatoux and M. Papuchon, “Progress in optical fiber gyroscopes using integrated optics”, Agard/Nato Conference on Guided Optical Structures in the Military Environment, AGARD CPP-383, 9A/1-13, 1985).
On the other hand, an inertial guidance or navigation system generally includes a three-gyroscope system so as to measure the rotations about three axes of a coordinate system in space, the axes of the three fiber coils forming the axes of the coordinate system. Let's note that, in some cases, for redundancy need, more than three axes may be used.
In such systems, for reasons of compactness, cost and stability, the optical technologies in solid medium are preferred. Hence, the FOGs implement the technologies of stimulated-emission amplified spontaneous emission source or ASE (Amplified Spontaneous Emission) source with a rare-earth doped amplifying optical fiber, or of semi-conductor superluminescent diode, of optical fiber (for the coil) and of integrated optical circuit (IOC) with multiple functions (polariser, beam splitter, phase modulators). FIG. 1 shows a three-FOG system including a single light source 10 and three interferometers. The source 10 generates a light beam 20. A first beam splitter 30 splits the beam 20 into three split beams. The beam splitter 30 is for example a 1×3 fiber-optic coupler. Each interferometer includes a fiber-optic coil 18, respectively 28, 38, and an integrated optical circuit 41, respectively 42, 43. The integrated optical circuits 41, 42, 43 allow performing several functions integrated on a same substrate. These circuits allow to increase the compactness of the device and to reduce the manufacturing costs. Typically, each integrated optical circuit 41, 42, 43 includes a single-mode waveguide at the input, a coil splitter, for example a Y-junction, an input connection towards the source and towards a detector, and two connections towards the ends of the fiber coil. Advantageously, each integrated optical circuit serves as a polariser and includes means for modulating the signal. The light source 10 is connected by single-mode optical fibers to each integrated optical circuit. Each fiber coil 18, respectively 28, 38, is connected to an independent integrated optical circuit on different substrates (cf. FIG. 1).
For reasons of compactness and cost, it is desirable to replace the different independent integrated optical circuits of the device of FIG. 1 by a multiple integrated optical circuit formed by the integration of several individual IOCs on a single substrate. A multiple integrated optical circuit may hence be advantageously connected on the one hand to a single and same source and on the other hand to different fiber-optic Sagnac ring interferometers. Moreover, the use of fiber comb (multi-fiber array) is known to simplify the alignment between the ends of optical fibers and of waveguides of a multiple integrated optical circuit. A fiber comb is generally consisted of a planar substrate including “V” notches regularly spaced from each other so as to position accurately the ends of different optical fibers along a same line and to allow a collective coupling.
FIG. 2 shows a top view of a multiple integrated optical circuit connected to optical fibers combs at the input and the output. By way of example, the multiple integrated optical circuit includes three individual integrated optical circuits arranged in parallel on the same substrate 2. In the example shown, each individual integrated optical circuit includes a coil splitter, of the Y-junction type, having a common trunk 12 (respectively 22, 32) and two branches 121 and 122 (respectively 221, 222 and 321, 322). A first fiber comb 51 allows to align and hold optical fibers 11, respectively 21, 31, opposite the ends of the common trunks 12, respectively 22, 32. A second fiber comb 52 allows to align and hold the optical fibers 71, 72, 73, 74, 75, 76 opposite the opposite ends of the different branches of the waveguides, respectively 121, 122, 221, 222, 321, 322. A beam propagating in the optical fiber 11 is coupled in the waveguide 12, then split by the Y-junction into two secondary beams propagating separately up to the ends 121 and 122 of the waveguide. The secondary beams are coupled to the ends of the optical fibers 71 and 72.
Moreover, an integrated optical circuit allows to integrate other functions on the same substrate: polarisation of the beams, modulations . . . . Advantageously, the common trunks 12, 22, 32 include polarising waveguides 14, 24, 34 integrated on a lithium niobate substrate 2. Electro-optic phase modulators may be arranged on either side of the branches 121, 122 (respectively 221, 222 and 321, 322) to modulate the phase of the interferometric signals.
However, in practice, the use of a multiple integrated optical circuit in a system of several fiber-optic gyroscopes states problems of spurious couplings between the different fibers connected and hence between the different interferometers. If these spurious couplings are coherent, this may induce unstable bias defects. Moreover, it is to be avoid that the different interferometers are in coherence with each other. Preferably, a single and same light source is used for the different interferometers. The source used is generally a broad-spectrum source, having a coherence length from a few tens of micrometers (for a superluminescent diode) to several hundreds of micrometers (for an ASE source). It is known that a broad-spectrum source may be considered as emitting wave trains that have for length the coherence length of the source. Now, in practice, the lengths of the different optical paths are not equal to each other to within such ten or hundred of micrometers. A length difference of several millimeters is sufficient a priori to cancel the phenomena of coherent coupling between the different interferometers.
The physical phenomena at the origin of the spurious couplings in a multiple IOC may be varied and are generally complex: coupling of a part of the incidence wave via the substrate between the optical fiber ends on the opposite faces of the substrate or at a Y-junction, multiple internal reflections in the substrate . . . .
Different solutions have been contemplated to reduce the spurious couplings in an integrated optical circuit. Hence, the patent document U.S. Pat. No. 5,321,779 proposes to form notches in the lower and upper faces of a IOC in order to attenuate the spurious internal reflections.
However, even using such notches, the performances of each FOG of a system including a multiple integrated optical circuit on a same substrate remain limited in terms of time stability. In practice, bias unstabilities of 10−5 to 10−6 radians in phase shift are observed.